Peer influence plays a central role in theories of adolescent problem behavior, has been identified as a key correlate of marijuana use across numerous studies, and is a primary focus of most adolescent drug prevention programs. However, limitations in the research on peers and substance use preclude drawing strong conclusions about the role of peer relationships as a contributing factor to youth marijuana use. This project will use innovative social network methods to address these limitations through the analysis of longitudinal friendship network data from the National Longitudinal Study of Adolescent Health (Add Health). Add Health is the largest study of adolescent friendship networks ever conducted, as well as the only one with a nationally representative sample. Over 90,000 adolescents (Grades 7-12) completed a baseline in-school questionnaire. Over 20,000 of these adolescents completed a more detailed baseline in-home interview and more than 14,000 participated in three follow-up interviews over a 12-year period. Friendship nominations were collected at both baseline and one year later, providing us with a rare opportunity to apply network analysis to the examination of peer influence and selection effects on youth marijuana use. Using Add Health data to examine the social context of marijuana use has distinct advantages, including: independent assessments of marijuana use by adolescents and their peers; a large, diverse, and nationally representative sample; a longitudinal design with excellent retention; and high participation rates at Waves I and II, resulting in a thorough assessment of school-based friendship networks. This project will address the following aims: Aim 1: To determine the extent to which friendship networks influence marijuana use (influence effects) and marijuana use influences friendship selection (selection effects) using longitudinal friendship data; and to evaluate the long-term impact of peer influences on marijuana use over time. A comparison of the standard group identification method (NEGOPY) and the novel p* method will demonstrate whether the latter perspective, which considers the building blocks of these peer groups, (rather than the extremes of best friend or the entire friendship block), provides a different, more sophisticated understanding of the mechanisms associated with social influence and selection, with implications for intervention approaches. Aim 2: To investigate how different types of peer influence (i.e., best friend, romantic partner, close friends, casual friends, friends of friends, entire school) are concurrently and prospectively associated with marijuana use, and to explore whether these associations are moderated by key network structure characteristics (i.e., size, density). Aim 3: To identify adolescents who are more resilient versus vulnerable to different types of pro-drug use peer influences (from Aims 1 and 2), focusing on characteristics from four domains: personal factors; school factors; family factors; and neighborhood factors. PUBLIC HEALTH RELEVANCE: This project will use longitudinal data from Add Health, the largest study of adolescent friendship networks ever conducted and the only one with a nationally representative sample, to examine peer influence and peer selection effects on youth marijuana use using multiple, complementary analytic approaches. The project will also investigate how different types of peer influence are associated with marijuana use over time, as well as identify youth who are more resilient versus vulnerable to different types of pro-drug peer influences.